# The transfer function of a small position Sensor

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GHADAH AL-OBAIDI on 28 Sep 2021
Commented: Star Strider on 4 Oct 2021
I have the below matlab code and I would like to find :
1. First Order Fit: try (polyfit), Find the Matlab function for fitting: P = WHAT_FUNCTION(WHAT TO FIT as what is x, WHAT TO FIT as what is y, ORDER); Force_Nm_Linear = P(1) * Displacement_d_mm + P(2);
2. Second Order Fit, same as above: what is x, y, ORDER P = WHAT_FUNCTION(WHAT TO FIT, WHAT TO FIT, ORDER); Force_Nm_2nd_Order = P(3) + P(2)*Displacement_d_mm + P(1)*Displacement_d_mm.*Displacement_d_mm;
Note: we have to set some of the coefficients to = 0 to check if it can be ignored:
P(3) = 0;
Force_Nm_2nd_two_Terms = P(3) + P(2)*Displacement_d_mm + P(1)*Displacement_d_mm.*Displacement_d_mm;
My Matlab code is::::::::::::::::
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear all;
close all;
clc;
Displacement_d_mm = [0, 0.08, 0.16, 0.24, 0.32, 0.4, 0.48, 0.52];
Force_Nm = [0, 0.578, 1.147, 1.677, 2.187, 2.648, 3.089, 3.295];
% Part A: First Order Fit:
% In this case, x is the displacement and y is the force.
% where d is the displacement and F the force
P = polyfit(Displacement_d_mm, Force_Nm, 1);
Force_Nm_Linear = P(1) * Displacement_d_mm + P(2); % F = 6.3221d + 0.089
% Part B:
Displacement_d_mm_b = [0, 0.32, 0.52];
Force_Nm_b = [0, 2.187, 3.295];
% Second Order Fit:
P = polyfit(Displacement_d_mm, Force_Nm, 2);
Force_Nm_2nd_Order = P(3) + P(2)*Displacement_d_mm + P(1)*Displacement_d_mm.*Displacement_d_mm;

Star Strider on 28 Sep 2021
If you actually want the transfer function, use the System Identification Toolbox functions —
Displacement_d_mm = [0, 0.08, 0.16, 0.24, 0.32, 0.4, 0.48, 0.52] % Output 'y(t)'
Displacement_d_mm = 1×8
0 0.0800 0.1600 0.2400 0.3200 0.4000 0.4800 0.5200
Force_Nm = [0, 0.578, 1.147, 1.677, 2.187, 2.648, 3.089, 3.295] % Input 'u(t)'
Force_Nm = 1×8
0 0.5780 1.1470 1.6770 2.1870 2.6480 3.0890 3.2950
Ts = 1; % Sampling Interval (Replace With Actual Value)
idd = iddata(Displacement_d_mm(:), Force_Nm(:), Ts)
idd = Time domain data set with 8 samples. Sample time: 1 seconds Outputs Unit (if specified) y1 Inputs Unit (if specified) u1
ff_sys = tfest(idd,1)
ff_sys = From input "u1" to output "y1": 5.26 --------- s + 29.14 Continuous-time identified transfer function. Parameterization: Number of poles: 1 Number of zeros: 0 Number of free coefficients: 2 Use "tfdata", "getpvec", "getcov" for parameters and their uncertainties. Status: Estimated using TFEST on time domain data "idd". Fit to estimation data: 77.7% FPE: 0.00339, MSE: 0.001541
figure
compare(idd, ff_sys) Experiment to get different results.
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Star Strider on 4 Oct 2021
As always, my pleasure!
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